This invention relates to a system for controlling the rotation of a spindle, and more particularly to a spindle rotation control system which is adapted to rotate the spindle of a machine tool at a commanded speed, to stop the spindle at a commanded position with a high accuracy, and to increase the rigidity at which the spindle is held when at rest.
There are machine tools known in the art which have an automatic tool changing function that allows machining to be performed automatically while a variety of tools are changed, also automatically. The tool changing operation proceeds as follows. First, a magazine holding a number of tools is revolved to bring a vacant tool holding portion of the magazine into position directly above a spindle mechanism. The spindle mechanism, which is grasping an old tool to be exchanged for a new one, is then projected forwardly, after which the magazine positioned above the spindle mechanism is lowered to permit the old tool to be received and grasped by the vacant tool holding portion of the magazine. The spindle mechanism is then retracted so that the old tool separates from the spindle, thus transferring the old tool to the magazine. Next, the magazine is revolved to bring a desired new tool into position in front of the spindle, and the spindle mechanism is projected forwardly to receive and grasp the new tool. Finally the magazine is raised away from the spindle to complete the tool change operation.
It is required in the tool change mechanism of the foregoing type that the fitting portions of the spindle and a tool be mated smoothly during the changing of tools. In other words, a predetermined point on the spindle must be stopped accurately at a predetermined rotational position. To this end, machine tools having the conventional automatic tool change function are provided with a photoelectric detector or with a limit switch mechanism for detecting the position of a key on the spindle. The arrangement is such that the spindle is brought to a stop at the predetermined rotational position by the application of a mechanical brake which is actuated in response to a signal from the detecting means. Since this method makes use of the mechanical brake, however, brake shoe wear results after long periods of use so that the spindle eventually cannot be stopped at the predetermined rotational position, thereby giving rise to occasions where the changing of tools cannot be performed smoothly. Avoiding this situation entails troublesome maintenance and inspection work as well as the frequent replacement of parts.
A proposed solution to the above problem is a servo control system that enables the spindle to be stopped accurately at a predetermined rotational position through electrical means without relying upon a mechanical brake. However, since the inertia of the DC motor that drives the spindle is very large, it is necessary to reduce the system gain to maintain system stability (i.e., in order to preclude such phenomena as spindle overshoot and hunting). Since reducing the system gain results in a diminished system stability when the motor is at rest, the DC motor and the spindle connected thereto tend to rotate when any external force or mechanically eccentric load is applied. This alters the position at which the spindle is stopped and prevents tools from being changed smoothly. Furthermore, if the above-mentioned servo system is applied to an apparatus such as a turning center that has a spindle indexing function, the spindle is likely to move during a cutting operation as a result of the low rigidity of the spindle. This makes it impossible to machine a workpiece accurately. It is conventional practice, therefore, to make use of mechanical means such as a pin to prevent spindle rotation, but this complicates both the operating procedure and the mechanism itself. Moreover, if a sensor such as a resolver or position coder is employed as a means for detecting the rotational position of the spindle in the foregoing servo control system, the result is a higher cost since prior art circuits for forming position deviation signals are complicated in construction and raise the total cost of the system.